1. Field of the Invention
This invention relates to a method of recognizing an irradiation field on a radiation image, and an apparatus for carrying out the method.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image having good image quality by use of the processed image signal have heretofore been known in various fields. For example, as such techniques, the applicant proposed various radiation image recording and reproducing systems which use stimulable phosphor sheets.
When a radiation image of an object, such as a living body, is recorded on a recording medium, such as X-ray film or a stimulable phosphor sheet, it is desirable that adverse effects of radiation upon the living body can be kept as small as possible. Also, if object portions not related to a diagnosis, or the like, are exposed to radiation, the radiation will be scattered by such portions to the portion that is related to a diagnosis, or the like, and the image quality will be adversely affected by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, an irradiation field stop made from lead, or the like, is often used in order to limit the irradiation field to an area smaller than the overall recording region of the recording medium, such that radiation may be irradiated only to that portion of the object, the image of which is to be used.
In cases where a radiation image is recorded on a recording medium, such as a stimulable phosphor sheet, by using an irradiation field stop, an image of an object, or the like, is recorded in a region (i.e., an irradiation field) inward from the aperture contour of the irradiation field stop. Also, a region outward from the aperture contour of the irradiation field stop (i.e., a region outside of the irradiation field) is not exposed to the radiation. Therefore, an irradiation field contour on the image, which contour corresponds to the aperture contour of the irradiation field stop, constitutes edge lines in the image.
In cases where an image signal is detected from the recording medium, on which an image has been recorded within only the irradiation field, and image processing is carried out on the image signal the image processing, such as gradation processing, may be carried out on only the image signal components of the image signal, which correspond to the region inside of the irradiation field. In this manner, the amount of the processing can be reduced markedly, the load of the processing can be kept small, and the processing speed can be kept high.
The region outside of the irradiation field is not exposed to the radiation. Therefore, in cases where the image is a negative image recorded on medical X-ray film, the image density of the region outside of the irradiation field becomes approximately lowest on the image. By way of example, when the medical X-ray film is set on a viewing screen and the transmission image is seen with light produced by a fluorescent lamp, the region having the lowest image density becomes the very bright region. Therefore, in particular, the portion of the irradiation field which is close to the region outside of the irradiation field, cannot be seen clearly due to dazzling effects of the bright region outside of the irradiation field.
Accordingly, in the radiation image recording and reproducing systems, a process for forcibly replacing the image signal values, which correspond to the region outside of the irradiation field, by image signal values representing the highest level of image density is carried out. The process is ordinarily referred to as the blackening process. In cases where the blackening process is carried out, the boundary line between the region, on which the blackening process is carried out, and the region, on which the blackening process is not carried out, coincides with the irradiation field contour on the radiation image. Therefore, it is necessary for the irradiation field contour to be recognized accurately in accordance with the image signal.
The irradiation field contour can be recognized by, for example, utilizing the characteristics in that the irradiation field contour constitutes the edge lines in the image, and finding a portion, at which the value of the image signal changes sharply.
Various techniques for determining the edge lines have heretofore been proposed. For example, with a technique proposed in U.S. Pat. No. 4,851,678, the edge lines corresponding to a rectangular aperture contour of an irradiation field stop are determined by (a) processing a radiation image signal, which represents a radiation image, along the horizontal direction of the radiation image, edge candidate points constituting an edge line being thereby detected, (b) totaling the thus detected edge candidate points along the vertical direction, the position of the edge line, which extends along the vertical direction, with respect to the horizontal direction being thereby found, (c) processing the radiation image signal along the vertical direction, edge candidate points constituting an edge line being thereby detected, and (d) totaling the thus detected edge candidate points along the horizontal direction, the position of the edge line, which extends along the horizontal direction, with respect to the vertical direction being thereby found. With a technique proposed in U.S. Pat. No. 4,967,079, the edge lines are determined by (a) detecting an edge candidate point from image signal values corresponding to positions along each of a plurality of radial directions, which extend from a predetermined point lying within the irradiation field toward ends of the radiation image, a plurality of edge candidate points being thereby detected with respect to the plurality of the radial directions, and (b) connecting the adjacent edge candidate points.
However, as for radiation images used for medical purposes, the number of picture elements constituting a single radiation image, i.e. the number of components of the image signal representing the single radiation image, is as large as 4.times.10.sup.6 (=2,000 picture elements.times.2,000 picture elements). Therefore, considerable time has heretofore been required to carry out the process for recognizing an irradiation field on the radiation image.